Gentlemen,
During WW1several tunnels were dug underneath German lines and huge quantities of explosive deposited in them, these were detonated with devestating results as still can be seen from the craters to this day. Hovever, I believe at least two or three did not explode and were left as the tide of war moved on but one actually went off in 1955 (I think). Given the amount of ordance still found on the French battlefields is it possible one of these mines could still go off?

Explosives don't always degrade with age. In some cases they just become more sensative. Keeps life "interesting" for the EOD types. Locations of some of these efforts often weren't well mapped or forgotten especially in earlier times. ACW explosives still occasionally kill or injure people here in the states. My understanding is that Europe has a much bigger problem with the stuff left over from WWI and WWII.

The French government (and probably the Belgian government as well) still maintains a department dedicated to removing and disposing of unexploded ordnance from the battlefields of France. Such stupendous amounts of artillery shells were fired on the Western Front during WW1 that large quantities of duds are still rising up from the earth as a consequence of seasonal freeze/thaw cycles. Farmers who turn them up during their Spring plowings pile their finds by the roadside where they are collected for disposal by the afore-mentioned government ordnance team, as are those duds discovered during construction and excavation projects. To this day, it is not unheard of for farmers in northern France and Belgium to be killed by the explosion of dud shells turned up by their plows. In addition, a lot of UXBs, dud ordnance, and abandoned ammunition caches are being turned up from WW2. Even dud artillery shells dating back to the Franco-Prussian War are occasionally still being discovered.

Both National Geographic and the Smithsonian magazine have featured articles on this topic. Smithsonian Magazine reported one notably sad story about a French family camping in the forest in the Verdun region (IIRC). They built a campfire atop what was later determined to have been a dud 5.9 or 6 inch howitzer shell hidden just below the surface of the ground. The shell exploded due to the heat of the campfire and killed the entire family.

A serious problem also exists with unexploded gas shells, which are extremely difficult to dispose of safely.

In the Ardennes region of France, large-scale citizen evacuations were necessary during MEC removal operations in 2001. In the forests of Verdun French government "démineurs" working for the Département du Déminage still hunt for poisonous, volatile, and/or explosive munitions and recover about 900 tons every year. The most feared are corroded artillery shells containing chemical warfare agents such as mustard gas. French and Flemish farmers still find many UXOs when ploughing their fields, the so-called "iron harvest".

In Belgium, Dovo, the country's bomb disposal unit recovers between 150 and 200 tons of unexploded bombs each year. Over 20 members of the unit have been killed since it was formed in 1919.